A method of separating an epitaxial semiconductor layer that has grown on a first substrate from the first substrate and moving the same onto a second substrate has been proposed as a method suitable for manufacturing a semiconductor device (see, for example, Japanese Patent Laying-Open No. 2001-053056 (hereinafter, referred to as Patent Document 1)).
A method disclosed in Patent Document 1 will be described hereinafter with reference to FIGS. 5A to 5D. Initially, referring to FIG. 5A, a first epitaxial GaN layer 52 and a mask 54 are successively formed on a sapphire substrate 51 serving as a first substrate. Here, a material that is unlikely to epitaxially grow, such as silicon dioxide (SiO2) or tungsten, is employed for mask 54. Mask 54 is provided so as to cover a part of a surface of first epitaxial GaN layer 52. Here, first epitaxial GaN layer 52 serves as a seed layer. Accordingly, after second epitaxial GaN layer 57 fills a window (opening) in mask 54, a continuous second epitaxial GaN layer 57 is formed on first epitaxial GaN layer 52 and mask 54.
Referring next to FIG. 5B, a trench (groove or recess) 60 extending through second epitaxial GaN layer 57 and exposing mask 54 is formed. Trench 60 is formed, for example, by etching second epitaxial GaN layer 57 so as to expose mask 54.
Referring next to FIG. 5C, a second substrate 58 is attached to an exposed surface of second epitaxial GaN layer 57. Second substrate 58 is bonded to second epitaxial GaN layer 57 with an already known wafer bonding method. Here, a conductive substrate composed of silicon (Si) is used as second substrate 58.
Thereafter, a chemical etchant is introduced in trench 60, so as to etch mask 54. Here, SiO2 is used for mask 54, while hydrofluoric acid (HF) is used as the chemical etchant. HF chemically attacks mask 54, and etches mask 54 at a rate greater than that in etching first epitaxial GaN layer 52, second epitaxial GaN layer 57 or second substrate 58. When mask 54 is etched away, second epitaxial GaN layer 57 is separated from sapphire substrate 51 and first epitaxial GaN layer 52.
Referring next to FIG. 5D, a polyimide material 59 fills trench 60, thus forming a semiconductor device.
According to the method described above, however, initially, first epitaxial GaN layer 52 is formed on the entire surface of sapphire substrate 51. Accordingly, warping of a wafer including the sapphire substrate, the GaN layer and the mask layer is caused due to difference in thermal expansion coefficient between the sapphire substrate and the GaN layer. Here, uniform adhesion in wafer bonding of second substrate 58 (Si substrate) to second epitaxial GaN layer 57 cannot be achieved because of such warping, and yield becomes very poor. In addition, crystal growth of the epitaxial GaN layer cannot be continuous. That is, the step of forming mask 54 between the first epitaxial GaN layer and the second epitaxial GaN layer is interposed, which means that crystal growth of the GaN layer is divided into two stages, resulting in expensive cost. Moreover, as trench 60 portion is removed and thrown away, the material is wasted and the cost is increased. Meanwhile, in the method of etching mask 54 with a chemical etchant, the mask is not sufficiently impregnated with the chemical etchant, because mask 54 is thin and a region of trench 60 is also thin and narrow. Accordingly, separation of the epitaxial layer from the sapphire substrate is difficult, and yield is poor.